1. Field of the Invention
The present invention relates to methods of producing golf balls, capable of forming a cover having a uniform thickness to produce a golf ball excellent in durability.
2. Description of the Conventional Art
Conventionally in producing a golf ball, an injection molding process or a heat and compression molding process is generally employed to place a cover on a core. In the injection molding process, at an initial stage of injection of a material for the cover, the core is held by a pin at the center of a cavity and when the injection enters a final stage, the pin recedes. As such, the injection molding process can result in the core having its center offset from the ball's center and tend to produce a cover having an uneven thickness. A golf ball having a cover with a large thickness (for example of 1.4 to 2.3 mm) often ensures that it has sufficient physical properties, even if injection molding provides a cover having an uneven thickness. If a cover has a small thickness (for example of 0.3 to 1.0 mm), however, and the thickness is uneven, which significantly and negatively affects the ball's performance. Injection molding has its limit when it is used as described above.
By contrast, the heat and compression molding process, as compared with the injection molding process, can form a cover to have a uniform thickness. Accordingly for a cover having a small thickness, the heat and compression molding process can be suitably employed.
U.S. Publication No. US 2003/0064828 A1 describes that in producing a solid golf ball including a cover having a thickness of 0.5 to 2.0 mm the cover's material or two half shells are compression molded to cover an intermediate layer. However, heat and compression molding the cover material to produce a solid golf ball, causes air to remain between the cover and the core, and the cover material excessively flows out of the mold, resulting in an insufficient thickness of the cover, and an uneven thickness of the cover.